diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0c72d37f1..1691091ec 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 - Fix password change with Directadmin driver (#8322, #8329)
 - Fix so css files in plugins/jqueryui/themes will be minified too (#8337)
 - Fix handling of unicode/special characters in custom From input (#8357)
+- Fix some PHP8 compatibility issues (#8363)
 
 ## Release 1.5.1
 
diff --git a/program/actions/mail/compose.php b/program/actions/mail/compose.php
index 1301780c2..3dc247d8d 100644
--- a/program/actions/mail/compose.php
+++ b/program/actions/mail/compose.php
@@ -689,6 +689,14 @@ class rcmail_action_mail_compose extends rcmail_action_mail_index
         return $body;
     }
 
+    /**
+     * Prepare message (or message part) body for composition
+     *
+     * @param rcube_message|rcube_message_part $part   Message or message part object
+     * @param bool                             $isHtml Use HTML mode
+     *
+     * @return string Message body text
+     */
     public static function compose_part_body($part, $isHtml = false)
     {
         // Check if we have enough memory to handle the message in it
diff --git a/program/actions/mail/search_contacts.php b/program/actions/mail/search_contacts.php
index 5061fa4ac..c9f7e70b4 100644
--- a/program/actions/mail/search_contacts.php
+++ b/program/actions/mail/search_contacts.php
@@ -87,8 +87,9 @@ class rcmail_action_mail_search_contacts extends rcmail_action_mail_list_contact
             // create javascript list
             while ($row = $result->next()) {
                 $name      = rcube_addressbook::compose_list_name($row);
-                $classname = $row['_type'] == 'group' ? 'group' : 'person';
-                $keyname   = $row['_type'] == 'group' ? 'contactgroup' : 'contact';
+                $is_group  = isset($row['_type']) && $row['_type'] == 'group';
+                $classname = $is_group ? 'group' : 'person';
+                $keyname   = $is_group ? 'contactgroup' : 'contact';
 
                 // add record for every email address of the contact
                 // (same as in list_contacts.inc)
diff --git a/program/actions/mail/send.php b/program/actions/mail/send.php
index ae199d797..b8edf16c9 100644
--- a/program/actions/mail/send.php
+++ b/program/actions/mail/send.php
@@ -366,6 +366,9 @@ class rcmail_action_mail_send extends rcmail_action
                 $is_inline    = preg_match($dispurl, $message_body);
             }
 
+            $ctype = isset($attachment['mimetype']) ? $attachment['mimetype'] : '';
+            $ctype = str_replace('image/pjpeg', 'image/jpeg', $ctype); // #1484914
+
             // inline image
             if ($is_inline) {
                 // Mail_Mime does not support many inline attachments with the same name (#1489406)
@@ -392,14 +395,13 @@ class rcmail_action_mail_send extends rcmail_action
                 }
 
                 if (!empty($attachment['data'])) {
-                    $message->addHTMLImage($attachment['data'], $attachment['mimetype'], $attachment['name'], false, $cid);
+                    $message->addHTMLImage($attachment['data'], $ctype, $attachment['name'], false, $cid);
                 }
                 else {
-                    $message->addHTMLImage($attachment['path'], $attachment['mimetype'], $attachment['name'], true, $cid);
+                    $message->addHTMLImage($attachment['path'], $ctype, $attachment['name'], true, $cid);
                 }
             }
             else {
-                $ctype   = str_replace('image/pjpeg', 'image/jpeg', $attachment['mimetype']); // #1484914
                 $file    = !empty($attachment['data']) ? $attachment['data'] : $attachment['path'];
                 $folding = (int) $rcmail->config->get('mime_param_folding');
 
diff --git a/program/include/rcmail_action.php b/program/include/rcmail_action.php
index a767fa3ac..110515513 100644
--- a/program/include/rcmail_action.php
+++ b/program/include/rcmail_action.php
@@ -683,10 +683,10 @@ abstract class rcmail_action
             header('Content-Type: ' . $file['mimetype']);
             header('Content-Length: ' . $file['size']);
 
-            if ($file['data']) {
+            if (isset($file['data']) && is_string($file['data'])) {
                 echo $file['data'];
             }
-            else if ($file['path']) {
+            else if (!empty($file['path'])) {
                 readfile($file['path']);
             }
         }
diff --git a/program/include/rcmail_sendmail.php b/program/include/rcmail_sendmail.php
index b94aa784d..e9337ecc2 100644
--- a/program/include/rcmail_sendmail.php
+++ b/program/include/rcmail_sendmail.php
@@ -1353,7 +1353,7 @@ class rcmail_sendmail
             $mdn_default = $_POST['_mdn'];
         }
         else if (in_array($this->data['mode'], [self::MODE_DRAFT, self::MODE_EDIT])) {
-            $mdn_default = (bool) $this->options['message']->headers->mdn_to;
+            $mdn_default = !empty($this->options['message']->headers->mdn_to);
         }
         else {
             $mdn_default = $this->rcmail->config->get('mdn_default');
diff --git a/program/lib/Roundcube/bootstrap.php b/program/lib/Roundcube/bootstrap.php
index 17b143631..6aaa444fa 100644
--- a/program/lib/Roundcube/bootstrap.php
+++ b/program/lib/Roundcube/bootstrap.php
@@ -113,10 +113,14 @@ PEAR::setErrorHandling(PEAR_ERROR_CALLBACK, function($err) { rcube::raise_error(
  */
 function in_array_nocase($needle, $haystack)
 {
+    if (!is_string($heystack)) {
+        return false;
+    }
+
     // use much faster method for ascii
     if (is_ascii($needle)) {
         foreach ((array) $haystack as $value) {
-            if (strcasecmp($value, $needle) === 0) {
+            if (is_string($value) && strcasecmp($value, $needle) === 0) {
                 return true;
             }
         }
@@ -124,7 +128,7 @@ function in_array_nocase($needle, $haystack)
     else {
         $needle = mb_strtolower($needle);
         foreach ((array) $haystack as $value) {
-            if ($needle === mb_strtolower($value)) {
+            if (is_string($value) && $needle === mb_strtolower($value)) {
                 return true;
             }
         }
diff --git a/program/lib/Roundcube/rcube_plugin_api.php b/program/lib/Roundcube/rcube_plugin_api.php
index 748673d55..ad305fd67 100644
--- a/program/lib/Roundcube/rcube_plugin_api.php
+++ b/program/lib/Roundcube/rcube_plugin_api.php
@@ -368,7 +368,7 @@ class rcube_plugin_api
             // read local composer.lock file (once)
             if (!isset($composer_lock)) {
                 $composer_lock = @json_decode(@file_get_contents(INSTALL_PATH . "/composer.lock"), true);
-                if ($composer_lock['packages']) {
+                if ($composer_lock && !empty($composer_lock['packages'])) {
                     foreach ($composer_lock['packages'] as $i => $package) {
                         $composer_lock['installed'][$package['name']] = $package;
                     }
@@ -376,7 +376,7 @@ class rcube_plugin_api
             }
 
             // load additional information from local composer.lock file
-            if (!empty($json['name']) && !empty($composer_lock['installed'])
+            if (!empty($json['name']) && $composer_lock && !empty($composer_lock['installed'])
                 && !empty($composer_lock['installed'][$json['name']])
             ) {
                 $lock            = $composer_lock['installed'][$json['name']];